Harry, I know that everyone assumes that Amazon has majority share of the market, but has anyone ever seen any actual figures.

Amazon don't report sale figures, but ask in the writers' forum here at MR. You'll find that pretty much everyone reports that the overwhelming majority of their sales (often 90%+) come via Amazon. Sales at the ePub bookstores are negligible by comparison.

Do a Google search for "Amazon ebook market share". You'll find that different analysts have different opinions, but they all say that Amazon have anywhere between a 70% and 90% share of the US eBook market, and their UK share is even higher, because there is no strong 2nd place player in the UK market, as B&N are in the US. Let's be generous and say that Amazon have only a 70% share of the combined US/UK ebook market. That still puts them considerably over 50% of global eBook sales.